High chromium martensitic heat-resistant steel with combined high creep rupture strength and oxidation resistance

ABSTRACT

Provided is martensitic heat-resistant steel for boiler applications with a unique combination of enhanced creep strength and excellent oxidation resistance upon high temperature exposure in steam containing environments contacts (in wt.-%): C: 0.10 to 0.16%, Si: 0.20 to 0.60%, Mn: 0.30 to 0.80%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 10.5 to 12.00%, Mo: 0.10 to 0.60%, V: 0.15 to 0.30%, Ni: 0.10 to 0.40%, B: 0.008 to 0.015%, N:0.002 to 0.020%, Co: 1.50 to 3.00%, W: 1.50 to 2.50%, Nb: 0.02 to 0.07%, Ti: 0.001-0.020%, iron and unavoidable impurities. The steel is normalized for about 10 to about 120 minutes at a temperature of 1050-1170° C. and cooled down in air or water to room temperature, and then tempered for at least one hour at a temperature of 750-820° C. The steel has a martensitic microstructure with average δ-ferrite content of less than 5 vol.-%.

The invention relates to martensitic high chromium heat-resistant steelsfor components operating at elevated temperatures i.e. between 550 and750° C. and high stresses. The steel according to the invention can beused in power generation, chemical and petrochemical industry.

STATE OF THE ART

The ferritic/martensitic high Cr steel materials are widely used in themodern power plants as reheater/superheater tubes and as steam pipes.Further improvement of the net efficiency of thermal power plants willrequire an increase of the steam parameters pressure and temperature.Therefore, the realization of more efficient power plant cycles willrequire stronger materials with improved steam-side oxidationresistance. The known efforts to develop new martensitic high chromiumsteel that combines excellent creep properties and superior oxidationsresistance have failed so far due to the formation of the so calledZ-phase. Z-phase is a complex nitride that coarsens quickly therebyconsuming the surrounding strengthening MX precipitates, M being: Nb, Vand X being: C, N.

The expression high chromium steel material generally means steels withmore than 9 wt.-% of Cr. Elevated Cr contents i.e. containing more than9 wt.-% of Cr, which are essential for good steam oxidation resistance,however, increase the driving force for Z-phase formation and alsoenhance the coarsening rate of chromium carbide precipitates. Both, theloss of the microstructure stabilizing effect of MX and chromium carbideprecipitates are responsible for the drop in the long-term creep rupturestrength of martensitic high Cr heat-resistant steel grades. Hence, themajor challenge for future steel developments is to resolve the apparentcontradiction between the creep rupture strength and oxidationresistance.

Currently, for high-temperature applications, that is applications withtemperatures of service higher than 550° C., ASTM Grades 91 and 92 arewidely used, both containing 9 wt.-% Cr with creep rupture strengthsafter 10⁵ h at 600° C. at 90 and 114 MPa respectively. The maindifference between the two steels is that Grade 92 contains W in therange of 1.8 wt.-% and reduced Mo of 0.4 wt.-% compared to 1 wt.-% incase of Grade 91. Additionally, Grade 92 contains small amounts of Bbelow 0.005 wt.-%.

Both steels suffer from insufficient oxidation resistance in steamatmospheres at temperatures above 600° C., which is limiting theapplication temperature range significantly. Especially in boilercomponents with heat transfer, the oxide scale acts as thermal insulatorthereby increasing the metal temperature and consequently reducinglifetime of corresponding components. Additionally, the oxide scales, ifspalled off during operation, will cause erosion damage on the followingsteam carrying components or after entering the steam turbine on turbineblades and guiding vanes. Spalled oxide scales may cause tube blockageespecially in the region of bends, impeding the steam flow oftenresulting in local overheating and catastrophic failure.

X20CrMoV11-1 is a well established high Cr ferritic/martensitic steelfor high temperature applications containing 0.20 wt.-% C, 10.5-12wt.-percent Cr, 1 wt.-% Mo and 0.2 wt.-% V. This steel exhibitsoxidation properties which are better than that of ASTM steel grades 91and 92 due to higher Cr contents, but poor creep rupture strength (creeprupture strength after 10⁵ h at 600° C. being around 59 MPa).Additionally the hot-workability and weldability are deteriorated due tohigh C content of 0.20 wt.-%. ASTM Grade 122 contains 10-12% Cr, 1.8% W,1% Cu and also V, Nb and N additions to induce the precipitation of MXstrengthening particles. The creep rupture strength is significantlybelow that of ASTM Grade 92 that presents a creep rupture strength of 98MPa after 10⁵ h at 600° C.

Also hot-workability issues due to elevated Cu contents are present.

Another steel with 11 to 12 wt.-% of Cr exists. it is mainly used asthin-walled tube, and is called VM12-SHCsteels that combines goodsteam-side oxidation resistance and the creep rupture strength at thelevel of ASTM Grade 91. Such steel concept is known from patentapplication WO02081766 disclosing a steel for high temperature usecontaining by weight: 0.06 to 0.20% of C, 0.10 to 1.00% of Si, 0.10 to1.00% of Mn, not more than 0.010% of S, 10.00 to 13.00% of Cr, not morethan 1.00% of Ni, 1.00 to 1.80% of W, Mo such that (W/2+Mo) is not morethan 1.50%, 0.50 to 2.00% of Co, 0.15 to 0.35% of V, 0.040 to 0.150% ofNb, 0.030 to 0.12% of N, 0.0010 to 0.0100% of B and optionally up to0.0100% of Ca, the rest of the chemical composition consisting of ironand impurities or residues resulting from or required for preparationprocesses or steel casting. The chemical constituent contents preferablyverify a relationship such that the steel after normalizing heattreatment between 1050 and 1080° C. and tempering has a temperedmartensite structure free or practically free of delta ferrite. Comparedto this steel, creep rupture strength can still be improved whilekeeping the other properties such as corrosion resistance and mechanicalproperties unaffected.

OBJECT AND SOLUTION

The object of the present invention is therefore to provide a seamlesstubular product in a martensitic heat-resistant steel with substantiallybetter creep rupture strength than ASTM Grade 92 steel for pipes andtubes, and with hot corrosion and steam oxidation behavior comparable orbetter than X20CrMoV11-1 and VM12-SHC steels, described in the state ofthe art.

A further object of the invention is to obtain a steel exhibitingmartensitic microstructure with a limitation of the delta ferrite, alsoknown as δ-ferrite, content to 5 vol.-% in average.

Another object of the invention was to provide a steel that allows thefabrication of small or large diameter seamless tubular products such asseamless tubes or seamless pipes, and a steel suitable for thefabrication of welded tubes and pipes, forgings and plates using theknown and established manufacturing processes.

The steel is suited as a production material for whole variety ofcomponents operating under stress at elevated temperatures, particularlyas seamless and welded tubes/pipes, forgings and plates in powergeneration, chemical and petrochemical industry. In addition, the steelaccording to the invention is temper resistant, after long temperingtimes up to 30 hours at 800° C., the yield strength is above or equal440 MPa, the tensile stress above or equal 620 MPa and toughness at 20°C. is above or equal 40 J when tested in longitudinal direction and 27 Jwhen tested in transverse direction.

In accordance with the present invention, the object can be achieved bya seamless tubular product for high-temperature applications in a steelhaving the following chemical composition in weight percent:

C: 0.10 to 0.16%

Si: 0.20 to 0.60%

Mn: 0.30 to 0.80%

P≤0.020%

S≤0.010%

Al≤0.020%

Cr: 10.50 to 12.00%

Mo: 0.10 to 0.60%

V: 0.15 to 0.30%

Ni: 0.10 to 0.40%

B: 0.008 to 0.015%

N: 0.002 to 0.020%

Co: 1.50 to 3.00%

W: 1.50 to 2.50%

Nb: 0.02 to 0.07%.

Ti: 0.001 to 0.020%, the balance of said steel being iron andunavoidable impurities.

Preferably, the ratio of boron and nitrogen is such that: B/N≤1.5 toachieve hot workability.

Preferably, the following equation is satisfied:

1.00%≤Mo+0.5W≤1.50% (in wt %),

In another preferred embodiment, the following equation is satisfied (inwt.-%):

B−(11/14)(N−10^(−(1/2.45)·(log B+6.81))−(14/48)·Ti)≥0.007

In another preferred embodiment, the following equation is satisfied (inwt.-%):

2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2

In a preferred embodiment, the carbon content is between 0.13 and 0.16%.

In another preferred embodiment, the Mo content is between 0.20 and0.60%.

Preferably, B content is between 0.0095 and 0.013%.

In a preferred embodiment, the Ti content is between 0.001 and 0.005%.

In another preferred embodiment, the microstructure comprises in averageat least 95% of tempered martensite, the balance being delta ferrite.

In an even more preferred embodiment, the microstructure comprises inaverage at least 98% of tempered martensite, the balance being deltaferrite.

In the most preferred embodiment, the microstructure is martensitic andfree of delta ferrite.

The invention also relates to a method of production comprising thefollowing steps:

-   -   casting a steel with a chemical composition according to the        invention,    -   hot forming said steel,    -   heating said steel and holding said steel for a time between 10        and 120 minutes in the temperature range between 1050° C. and        1170° C.,    -   cooling said steel down to room temperature,    -   reheating said steel and holding said steel up to a tempering        temperature TT that is between 750° C. and 820° C. for at least        one hour,    -   cooling said steel down to room temperature.

Preferably, the cooling step is done using air cooling or water cooling.

The cooling step after reheating step may be done using water cooling.

The cooling step after heating step may be done using water cooling.

The invention may also concern the production of a welded tube, pipe orplate using the same steel as the one according to the seamless tubularproduct of the invention or the process according to the invention.

FIG. 1 shows the schematic of mass gain due to oxidation plotted versuschromium content.

SUBJECT MATTER OF THE INVENTION

In accordance with the present invention, a martensitic high chromiumheat-resistant steel is created having the following chemicalcomposition:

(1) C: 0.10 to 0.16%,

C needs to be added to at least 0.10% to obtain sufficient carbideprecipitation. Additionally C is also an austenite stabilizing element.C contents below 0.10% would imply more δ-ferrite in the microstructure.The upper limit for carbon is 0.16% because excess C addition limits thetoughness and weldability properties.

(2) Si: 0.20 to 0.60%,

Si is used for deoxidation during the steel making process.Additionally, it is one of key elements, which determines the oxidationbehavior in steels. In order to achieve the full oxidation improvingeffect of Si additions an amount of at least 0.20% is necessary. Theupper Si level shall preferably be limited to 0.60%, because the excessSi addition accelerates the coarsening of precipitates and decreasestoughness. Preferably the lower limit is 0.25%.

(3) Mn: 0.30 to 0.80%,

Mn is an effective deoxidation element. It ties up sulphur and reducesthe δ-ferrite formation. At least 0.30% Mn may be added. The upper limitshall be 0.8%, since excessive additions reduce the strength of steelsat elevated temperatures.

(4) P≤0.020%,

P is a grain-boundary active element, which reduces the toughnessproperties of steels. The content has to be limited to 0.020% in orderto avoid the negative impact of P on toughness properties. P may bepresent in an amount equal to or greater than 0.00% as it may beunavoidable as an impurity.

(5) S≤0.010%,

S forms sulfides and reduces the toughness and hot-workabilityproperties of steels. A limitation of upper S content to 0.010 preventsthe defect formation during hot-working operation and the negativeimpact on toughness. S may be present in an amount equal to or greaterthan 0.00% as it may be unavoidable as an impurity.

(6) Al≤0.020%,

Al is a potent deoxidation element used during the steel making process.Excess Al addition above 0.02% can induce AlN formation, therebyreducing the amount of strengthening MX (M being: Nb, V and X being: C,N) nitride precipitates in steel and consequently the creep strengthproperties. Al may be present in an amount equal to or greater than0.00% as it may be unavoidable as an impurity.

(7) Cr: 10.5 to 12.00%,

Cr forms carbides that form at boundaries of the martensiticmicrostructure. Chromium carbides are essential for stabilization of themartensitic microstructure during exposure at elevated temperatures. Crimproves the high temperature oxidation behavior of steels. Contents ofat least 10.5% are necessary to unfold the full oxidation improvingeffect of Cr additions. Cr contents above 12% result in increasedδ-ferrite formation.

(8) Mo: 0.10 to 0.60%,

Mo is an important element for improvement of creep rupture strengththat is also responsible for solid solution strengthening. This elementis incorporated in carbides and intermetallic phases as well. Mo contentof 0.10% may be added. The Mo additions above 0.60% will deterioratetoughness and induce increase of δ-ferrite content. Note that M and Wcontents shall satisfy the relationship (in weight %) 1≤Mo+0.5×W≤1.5, inorder to ensure the sufficient precipitation of carbides andintermetallic phases.

(9) V: 0.15 to 0.30%,

V combines with N to form coherent MX nitrides (M being: Nb, V and Xbeing: C, N), which contribute to enhancement of long-term creepproperties. Contents below 0.15% are not sufficient to achieve thislong-term creep improving property effect while contents above 0.30%decrease the toughness and increase the danger for δ-ferrite contentsabove 5% in average volume.

(10) Ni: 0.10 to 0.40%,

Ni is an important toughness improving element. Therefore, a minimumcontent of 0.10% is necessary. However, it reduces A_(c1) temperatureand tends to reduce the creep rupture strength, if added in contentsabove 0.40%.

(11) B: 0.008 to 0.015%,

B is a decisive element responsible for stabilization of M₂₃C₆ carbidesand delay of recovery of the martensitic microstructure. It strengthensthe grain boundaries and improves the long-term stability of creeprupture strength. In addition, B is responsible for remarkableimprovement of creep rupture ductility. For achievement of maximumstrengthening effect additions of at least 0.008% are necessary.Contents above 0.015%, however, reduce substantially the maximumprocessing temperature of steels and are regarded as detrimental. B andN additions shall satisfy the relationship B/N≤1.5 to enabletransformation using known hot-working processes. Indeed, this B/Nrelationship allows the fabrication of small or large diameter seamlessand welded tubes, pipes and plates using manufacturing process accordingto the invention. Preferably, the B content should be between 0.0095 and0.0130 (wt %).

(12) N: 0.002 to 0.020%,

Nitrogen is necessary for formation of MX (M being: Nb, V and X being:C, N) nitrides and carbonitrides responsible for achievement of creeprupture strength. At least 0.002% may be added. Excessive N additionsi.e. above 0.020%, however, result in enhanced BN formation, therebyreducing the strengthening effect of B additions.

Preferably, B and N contents (in weight %) shall satisfy the followingrelationship:

B−(11/14)(N−10^(−(1/2.45)·(log B+6.81))−(14/48)·Ti)≥0.007

(13) Co: 1.50 to 3.00%,

Co is a very effective austenite forming element and useful in limitingδ-ferrite formation. Moreover, it has only a weak effect on A_(c1)temperature. Additionally, it is an element that improves creep strengthproperties by reducing the size of initial precipitates after heattreatment. Therefore, a minimum content of 1.50% shall be added.Preferably the minimum content is 1.75%. However, Co in excessiveadditions may induce embrittlement due to enhanced precipitation ofintermetallic phases during high temperature operation. At the same timeCo is very expensive. Hence, a limitation of additions to 3.00%,preferably to 2.50%, is necessary.

It is preferable that the Ni, Co, Mn, C and N contents (in weight %) arein accordance with the following equation:2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2·(14) W: 1.50 to 2.50%,

W is known as an effective solution strengthener. At the same time it isincorporated in carbides and forms C14 Laves phase, which may contributeto creep strength enhancement as well. Therefore, a minimum content of1.50% is needed. However, this element is expensive, stronglysegregating during steel making and casting process and it formsintermetallic phases that lead to significant embrittlement. Hence, theupper limit for W additions may be set to 2.50%. Note that Mo and Wcontents (in weight %) shall satisfy the relationship 1.00≤Mo+0.5W≤1.50in order to ensure the sufficient precipitation of carbides andintermetallic phases.

(15) Nb: 0.02 to 0.07%.

Nb forms stable MX carbonitrides important not only for creep propertiesbut also austenite grain size control. A minimum content of 0.02% may beadded. Nb contents above 0.07% result in formation of coarse Nb carbidesthat may reduce the creep strength properties. Therefore the upper limitis set to 0.07%.

(16) Ti: 0.001-0.020%

Ti is a strong nitride forming element. It is helpful to protect free Bby forming nitrides. Minimum content of 0.001% is needed for thispurpose. Excessive Ti content above 0.020%, however, can reducetoughness properties due to formation of large blocky TiN precipitates.

The balance of the steel comprises iron and ordinary residual elementscoming from steel making and casting process. The casting techniquesused are the one known from the skilled man. By impurities we meanelements such as tantalum, zirconium and any other elements that can'tbe avoided. It is to be mentioned that Tantalum and zirconium are notintentionally added to the steel, however may be present in less than 50ppm overall as unavoidable impurities.

In an embodiment of the steel, the unavoidable impurities may compriseone or more of copper (Cu), Arsenic (As), tin (Sn), antimony (Sb) andlead (Pb).

Cu may be present in a content equal or less than 0.20%.

Element As may be present in a content equal or less than 150 ppm; Snmay be present in a content equal or less than 150 ppm; Sb may bepresent in a content equal or less than 50 ppm; Pb may be present in acontent equal or less than 50 ppm and the total content As+Sn+Sb+Pb isequal or less than 0.04% in mass.

The steel is normalized for a period of about 10 to about 120 minutes inthe temperature range between 1050° C. and 1170° C. and cooled down inair or water to room temperature, and then tempered for at least onehour in the temperature range between 750° C. and 820° C.

It has been found out that the resulting steel possesses remarkable andabsolutely excellent elevated temperature strength and superiorsteam-oxidation resistance. Moreover, it was found that byCr_(eq.)/Ni_(eq.) ratio being less than 2.3, the average δ-ferritecontent can be limited to less than 5 vol. % to avoid toughness issues,wherein Cr_(eq). and Ni_(eq). are defined as Cr+6Si+4Mo+1.5W+11V+5Nb+8Tiand 40C+30N+2Mn+4Ni+2Co+Cu, respectively. Surprisingly, it was foundthat the B/N ratio equal or less than 1.5 has to be kept in order toenable the hot-working operation with known transformation processes.

The delta ferrite content shall not exceed 5 vol.-% since contents above5 vol.-% will impair the toughness properties.

By hot forming processes, it is meant: hot rolling, pilgering, hotdrawing, forging, plug mill, push-bench process where the mandrel rodpushes the elongated hollow through several in-line roll stands toproduce a hollow, continuous rolling, and other rolling processes known.The steel according to the invention is able to be formed in the shapeof tubes and pipes. Numerous attempts have been made with steelsexhibiting satisfactory properties such as oxidation behavior, creepresistance but these steels failed in giving a satisfactory formedproduct through these hot forming processes. In particular, it was evensometime not possible to obtain seamless tubes or pipes. The steel ofthe invention enables having seamless tubular products with satisfactoryproperties and the possibility of obtaining seamless tubular products orplates by hot forming processes, these products being into dimensionalrequirements.

EXAMPLES

The benefits of the steel of the present invention will be explained inmore detail on the basis of the following examples. Steels in accordancewith the present invention (Steel 1, Steel 2, Steel 3) and alsocomparative example steels (Steel 4, Steel 5), having the chemicalcomposition indicated in Table 1, have been cast to 100 kg ingots usingvacuum induction melting furnace, then hot-rolled to plates (13-25 mmthickness) and subsequently normalized and tempered. The normalizingheat-treatment was performed in the temperature range of 1060° C. to1100° C. for 30 minutes, followed by air cooling to room temperature.The tempering was done at 780° C. for 120 minutes, again followed bycooling in air.

Comparative example steels 4 and 5 have B contents below 0.008 and aretherefore not in accordance with the invention.

In case of steel 4, the Ni, Co, Mn, C and N additions do not comply withequation

2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2 (in wt.-%).

The steel 5 does not fulfill the following formula:

B−(11/14)(N−10^(−(1/2.45)·(log B+6.81))−(14/48)·Ti)≥0.007 (in wt. %)either.

TABLE 1 Steel 1 Steel 2 Steel 3 Steel 4* Steel 5* Element (wt. %) (wt.%) (wt. %) (wt. %) (wt. %) C 0.15 0.148 0.148 0.158 0.152 Si 0.39 0.520.29 0.49 0.39 Mn 0.3 0.67 0.65 0.42 0.35 P 0.001 0.015 0.015 0.0050.001 S 0.002 0.001 0.002 0.001 0.002 Al 0.007 <0.002 0.007 0.007 0.006Cr 11.19 11.4 11.3 11.36 10.85 Mo 0.49 0.46 0.25 0.31 0.49 V 0.27 0.210.2 0.25 0.25 Ni 0.3 0.25 0.3 0.23 0.31 B 0.0145 0.011 0.0100 0.00400.0052 N 0.011 0.0088 0.0103 0.042 0.015 Co 1.77 1.9 1.9 0.88 1.72 W1.91 1.6 1.8 1.46 1.95 Nb 0.048 0.038 0.033 0.038 0.043 Ti 0.001 0.0030.001 0.001 0.001 *Comparitive steels

For the two example steels (Steel 1, Steel 2, Steel 3) the resultspresented in table 2 were obtained at room temperature for tensilestrength, yield stress, elongation, reduction of area and Charpy V notchimpact energy.

TABLE 2 Steel 1 Steel 2 Steel 3 P92 R_(p0.2) (MPa) 653 683 682 540 R_(m)(MPA) 840 855.5 859.5 710 A₆ (%) 20.5 22 21 23 Z (%) 64 64 60 65A_(v iso) (J) - RT 72 52 56 140

Creep tests, performed in accordance to ISO DIN EN 204, on the specimensof the two example steels showed furthermore a remarkable improvement ofthe creep rupture strength. This is reflected in rupture times being atleast almost two times more than that of state-of-the-art steels likeP91, P91, VM12-SHC, P122 and X20CrMoV11-1 during long-term creep testingat 130 MPa and 100 MPa. The results are displayed in Table 3. Also thecomparative example steels does not reach the creep rupture strength ofthe steels according to the invention.

TABLE 3 Rupture time in h at 650° C. for stresses Steel grade 130 MPa100 MPa Steel 1 6470 23844 Steel 2 1824 13867 Steel 3 2194 7552 Steel 4not tested 5900 Steel 5 526 3354 VM12-SHC 517 2828 P91* 44 498 P92* 6864682 P122 (single phase)** 533 4572 X20CrMoV11-1* 55 210 *Average valuescalculated from strength values indicated in ECCC data sheet **K. Kimuraet al.. Proc. of ASME PVP Conference (PVP2012), 2012, Toronto, Canada

FIG. 1 shows the schematic of mass gain due to oxidation in water vaporatmosphere at elevated temperatures plotted versus chromium content. Thebasis for the construction of the schematic is the oxidation tests inwater vapor atmosphere performed according to ISO 21608:2012.

In the FIG. 1, three regions displaying different steam oxidationbehavior have been defined as follows:

(I.) Non-protective behavior for mass gain above 10 mg/cm² after 5,000 h

(II.) Intermediate behavior for mass gain in the range 5-10 mg/cm²

(III.) Protective behavior for mass gains below 5 mg/cm².

Correspondingly, the classification of different high Cr martensiticheat-resistant steels with respect to oxidation behavior was performedin the table 4 below. Regions I, II and Ill correspond to mass gains asdescribed in FIG. 1. The two example steels clearly outperform P91, P92,P122 and X20CrMoV11-1 with respect to steam oxidation resistance. Theinvention exhibits behavior comparable to VM12-SHC.

TABLE 4 Mass gain (mg/cm²) Test temperature (° C.) 600° C. 650° C.VM12-SHC III III P92 I I X20CrMoV11-1 III I P122 (single phase) III IIInvention III III

According to the invention it is possible to provide a high chromiummartensitic heat-resistant steel with enhanced creep properties andsteam oxidation resistance that can be used to produce tubes, forgings,pipes and plates operating at high temperature in the power generation,chemical and petrochemical industry.

1. A seamless tubular product, made of a steel comprising, in weight percent: C: 0.10 to 0.16%, Si: 0.20 to 0.60%, Mn: 0.30 to 0.80%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 10.50 to 12.00%, Mo: 0.10 to 0.60%, V: 0.15 to 0.30%, Ni: 0.10 to 0.40%, B: 0.008 to 0.015%, N: 0.002 to 0.020%, Co: 1.50 to 3.00%, W: 1.50 to 2.50%, Nb: 0.02 to 0.07%, Ti: 0.001 to 0.020%, iron and unavoidable impurities.
 2. The seamless tubular product according to claim 1, wherein: B/N≤1.5.
 3. The seamless tubular product according to claim 1, wherein, the contents of Mo and W in wt % satisfy: 1.00%≤Mo+0.5W≤1.50%.
 4. The seamless tubular product according to claim 1, wherein the contents of B, N, and Ti in wt % satisfy: B−(11/14)(N−10^(−(1/2.45)·(log B+6.81))−(14/48)·Ti)≥0.007.
 5. The seamless tubular product according to claim 1, wherein, the contents of Ni, Co, Mn, C, and N in wt.-% satisfy: 2.6≤4·(Ni+Co+0.5·Mn)−20·(C+N)≤11.2.
 6. The seamless tubular product according to claim 1, wherein the carbon content is between 0.13 and 0.16%.
 7. The A seamless tubular product according to claim 1, wherein the Mo content is between 0.30 and 0.60%.
 8. The seamless tubular product according to claim 1, wherein the B content is between 0.0095 and 0.013%.
 9. The seamless tubular product according to claim 1, wherein the Ti content is between 0.001 and 0.005%.
 10. The seamless tubular product according to claim 1, wherein the product has a microstructure comprising at least 95% of tempered martensite, the balance being delta ferrite.
 11. The seamless tubular product according to claim 10, wherein the microstructure comprises at least 98% of tempered martensite.
 12. The seamless tubular product according to claim 10, wherein the microstructure is martensitic and free of delta ferrite.
 13. The seamless tubular product according to claim 1, which is a seamless tube.
 14. A method of producing the seamless tubular product according to claim 1, the method comprising: i) casting said steel, ii) hot forming said steel, iii) heating and holding said steel for a time between 10 and 120 minutes at a temperature ranging between 1050° C. and 1170° C., iv) cooling said steel down to room temperature, v) reheating and holding said steel up to a tempering temperature TT that is between 750° C. and 820° C. for at least one hour, and vi) cooling said steel down to room temperature.
 15. The method according to claim 14, wherein the cooling iv) and vi) are done using air cooling or water cooling. 